This will be the first marriage for Bledel, 31, and Kartheiser, 33.
People and Us Weekly confirmed the news, and Bledel's pal told Us Weekly that "they couldn't be happier."
"Vincent proposed a few weeks ago. She's been wearing her ring. It's huge!"
Bledel, who starred in the beloved "Gilmore Girls" series, dated costar Milo Ventimiglia for three years before splitting in 2006. Kartheiser previously dated "She's All That" star Rachel Leigh Cook, but the couple called it quits in 2010.
Their "Mad Men" characters became entangled in an affair last season when Kartheiser's Pete Campbell, an ad exec, took Bledel's mentally unstable Beth Dawes on as a mistress after she seduced him. In the finale, the two sleep together just before her character undergoes shock treatment.
The engagement announcement sort of comes at a perfect time for the couple to promote the show, since the sixth season of the Emmy-winning series begins April 7.
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